Motor speed sensors are widely used in a machine. Reliable and precise speed measurements are important for machine controls, such as traction control, wheel slide protection, registration, train control, door control, and so on. The electronic control module (ECM) of a machine typically includes an un-commanded motion detection (UCMD) function to detect faults and perform diagnostics on a motor speed sensor, when a fault is detected associated with the sensor. However, there is a potential that the motor speed sensor failure may not be successfully detected and notified to the operator. As a result, the undetected motor speed sensor failure may indirectly disable the UCMD function.
One instance of sensor detection failure occurs when an electrical wiring fault exists in a machine. For example, a parked machine experiences a sensor power supply line harness failure and stops supplying power to the motor speed sensor. When such a motor speed sensor failure occurs, the ECM may not receive a speed signal and may not know that the machine is moving. Accordingly, the ECM may not detect the motor speed sensor failure and the UCMD function may not be activated.
It is a regulatory requirement that an operator needs to be notified when a speed sensor fails and the failure detection (e.g., UCMD) ability is lost. Therefore, it becomes important to indirectly detect the machine movement when a motor speed sensor fails, to comply with the machine regulations. Additionally, it is also important to determine that the speed sensor detection function has failed and to notify the operator regarding the failure.
A method for calibrating the control output using a secondary reference in the event of speed sensor failure is described in U.S. Patent Publication No. 2007/0119136 to MacGregor et al. (“the '136 publication”). The '136 publication describes a technique called valve profiling. The technique calibrates the control output so that the control module has a secondary reference point as to how the pulse width modulation (PWM) valve should function in the event of a speed sensor failure. The method includes measuring set point PWM voltage/current values, and comparing the set point values with stored values in a look-up table. In the look-up table, the stored PWM voltage/current values may correspond to a set of speed sensor values. The method may further include determining a virtual speed based on the look-up table.
Although the technique described in the '136 publication may be effective for indirectly detecting machine movement, it may be problematic. For example, the calibration method described in the '136 publication requires that the ECM be able to measure reliable PWM voltage and current values to determine a motor speed. However, when un-commanded machine motion is present on a stopped machine, the PWM voltage/current values may not be accurately measurable, and consequently the virtual speed values obtained from the look-up table may not accurately indicate true motor speed values. In particular, the method described in the '136 publication may not be capable of detecting an un-commanded machine motion. Furthermore, although the method described in the '136 patent may indicate whether a fault exists on a speed sensor, it may be incapable of notifying an operator of the machine and activating diagnostics, once a motor speed sensor fault is detected.
The disclosed system and method for indirectly detecting machine movement is directed towards overcoming one or more of the shortcomings set forth above.